Cluster for adjusting a nuclear reactor core reactivity, absorber rod of the cluster and method for protecting the absorber rod against wear

ABSTRACT

A control cluster for a pressurized water nuclear reactor comprising a bundle of neutron-absorbing rods each of which comprises a metal tube called cladding which is sealed off by a top end plug at its top end and by a bottom end plug at its bottom end and has a support, or spider, of radiating shape to which the absorber rods are attached through their upper end plugs, characterized in that the cladding of at least some of the absorber rods is weld-free hafnium tubes, the top end plugs of the absorber rods having hafnium cladding being of titanium-based alloy and being welded to the top end part of the hafnium cladding of the absorber rod, the bottom end plugs being of massive hafnium and being welded to the bottom

FIELD OF THE INVENTION

The invention relates to a cluster for adjusting the reactivity of apressurized-light-water-cooled nuclear reactor core and an absorber rodin such an adjusting cluster.

BACKGROUND INFORMATION

Nuclear reactors such as pressurized water reactors comprise a coreconsisting of fuel assemblies placed adjacent to each other within thereactor vessel. A fuel assembly comprises a bundle of fuel rods held ina supporting structure called a skeleton assembly which comprises theframe for the assembly. This skeleton assembly in particular includesguide tubes located in the axial direction of the fuel assemblyconnecting the top and bottom ends and supporting the spacer grids forthe fuel rods. The purpose of these guide tubes is to ensure that theframework has satisfactory rigidity and to allow the assembly ofneutron-absorbing rods used to control the reactivity of the nuclearreactor core to be inserted.

The absorber rods are connected together at their top ends by a supportwhich is generally called a “spider”, to form a bundle called a controlcluster. The set of absorber rods can move within the guide tubes of thefuel assembly.

In order to regulate the reactivity of the nuclear reactor core whilethe reactor is in operation the vertical positions of the controlclusters within particular assemblies of the core are changed, either sothat they are inserted, the control cluster being then moved downwards,or extracted, the control cluster being then moved upwards, so that avariable length of absorber rod is inserted into the core assemblies.Control clusters of different types are generally used in differentparts of the nuclear reactor core to control the core's reactivity andthe power distribution within the reactor core while the nuclear reactoris in operation. Highly absorbent clusters, black clusters, and lessabsorbent clusters, grey clusters, are used in particular.

In general the absorber rods comprise a tube closed at its upper end bya first end plug called a top end plug and at its bottom end by a secondplug called a bottom end plug for the rod. The absorber rods are securedto the holding spider through their top end plugs.

Generally, in the case of black clusters the rod assembly comprises rodshaving a high neutron absorption capacity. These absorber rods maycomprise a cladding tube enclosing pellets of an absorbent material suchas boron carbide B₄C, tubes of a neutron-absorbing material which do notenclose absorbent pellets, or again tubes of absorbent materialenclosing pellets of boron carbide B₄C. In particular it has beensuggested that hafnium tubes should be used as tubes of absorbentmaterial for the rods in control clusters. Clusters adjusting thereactivity of nuclear reactors may therefore consist wholly or in partof absorber rods comprising a hafnium tube which may include pellets ofan absorbent material such as B₄C. In some circumstances it has beensuggested that only a part of the absorber rods, for example the bottompart, should be made of hafnium.

Grey clusters include both absorber rods and inert rods consisting of asimple tube of a material which is not absorbent or has littleabsorbency, closed by end plugs at its extremities. Absorber rods maycomprise tubes of absorbent material such as hafnium.

Hafnium has the advantage over other absorbent materials that it hasexcellent compatibility with the primary fluid, shows little swellingunder irradiation and has good creep resistance at the operatingtemperature of a pressurized water nuclear reactor. It can therefore beused without any sheathing.

However, hafnium can only be welded to alloys of the same family(titanium, zirconium, hafnium) or alloys forming continuous solidsolutions with hafnium.

If hafnium is used for the top end plug, the mechanical strength of thecontrol cluster is not optimal because hafnium does not havesufficiently good mechanical properties for the stresses experienced bythe cluster when in operation. Furthermore, the use of a hafnium plug inthe top part of an absorber rod is not really justified on the groundsof neutron absorption, given that the top plug is only exposed to a verylow neutron flux because it remains above the top of the core. Finally,the use of hafnium for the top end plug is accompanied by an increase inthe mass of the cluster, and this may constitute a strong operationalconstraint. The use of zirconium alloy for the top end plug would becompatible with the mass requirements without any deterioration inabsorbency. However, the mechanical properties of these alloys are alsoinadequate. Conversely, the properties of titanium alloys are perfectlycompatible with the performance required.

As far as the bottom end plug is concerned, the use of hafnium is notruled out for mechanical strength reasons because the properties of thismaterial are compatible with the mechanical stresses applied to thatcomponent. In this area where there is a high neutron flux it is usefulto have neutron absorption capacity. Finally as the volume of the bottomend plug is small, the resulting increase in mass is small andcompatible with the requirements for the mass of control clusters. Thebottom end plug may therefore be made of hafnium, or a zirconium alloy,while remaining compatible with functional requirements.

SUMMARY

The objective of the invention is therefore to provide a control clusterfor a pressurized water nuclear reactor comprising a bundle ofneutron-absorbing rods each of which comprises a metal tube calledcladding which is sealed off by a top end plug at its top end and by abottom end plug at its bottom end and has a support, or spider, ofradiating shape to which the absorber rods are attached through theirupper end plugs, characterised in that the cladding of at least some ofthe absorber rods is weld-free hafnium tubes, the top end plugs of theabsorber rods having hafnium cladding being of titanium-based alloy andbeing welded to the top end part of the hafnium cladding of the absorberrod, the bottom end plugs being of massive hafnium and being welded tothe bottom end of the hafnium cladding of the absorber rod.

In an example embodiment,

the top end plugs of absorber rods having hafnium cladding are made ofTA6V or TA3V2.5 titanium alloy,

protection against wear of the rods is provided by a flow of oxidisingatmosphere in a travelling arrangement to the cladding welded to thebottom end plug,

protection against wear of the top end plugs made of titanium alloy isobtained by static furnace treatment in an oxidising atmosphere underconditions ensuring that the properties of the alloy are maintained,

the welds for at least one of the top and bottom end plugs are madeusing one of the following procedures: friction welding, resistancewelding, TIG welding, and

the hafnium used for manufacture of the cladding and the bottom endplugs contains more than 300 ppm of oxygen.

The invention also relates to an absorber rod of a cluster adjusting apressurized water nuclear reactor characterised in that it comprises ahafnium tube, a titanium alloy top end plug welded to the top extremityof the hafnium tube and a bottom end plug of massive hafnium welded tothe bottom extremity of the hafnium tube.

Finally, the invention also relates to a cluster for adjusting apressurized water nuclear reactor comprising a bundle of rods and asupport of radiating shape, called a spider, to which the absorber rodsare attached through their top end plugs, characterised in that thespider is made of titanium-based alloy.

In an example embodiment, at least some of the absorber rods in thecluster comprises a hafnium tube and a top end plug of titanium alloywelded to the top extremity of the hafnium tube.

Hafnium tubes or hollow bars are prepared in accordance with a processby drawing pierced billets on a needle, and then hot drawing on adeformable mandrel, the mandrel being removed in the final operation bycold drawing up to failure. The advantage of this hot shaping process isthat it makes it possible to use metal with a very much higher oxygencontent than it should have for cold forming operations. It is generallyfelt that above 300 ppm of oxygen hafnium can no longer be cold rolled.This process makes it possible to use billets containing more than 300ppm and even more than 700 ppm of oxygen, as obtained after first fusionby electron bombardment in the conventional method of preparation. Theoxygen concentration makes it possible to increase the mechanicalproperties of the metal, which considerably reduces sensitivity tosurface and manufacturing defects (marks, out-of-alignment, etc.).

Zirconium or hafnium end plugs are obtained by the machining of solidbars of suitable diameter. This design makes it possible to satisfy theneutron, mechanical and weight requirements.

However, longitudinal and orbital movements of the cluster are likely togive rise to wear in the cluster guides (continuous guides and guidecards) and the fuel assembly (wear on the tip). It is known that thesematerials (titanium, zirconium and hafnium) do not withstand wear well.One known way of protecting these materials against wear is to providehigh temperature oxidation treatment in an oxidising atmosphere. Suchtreatment produces an oxygen diffusion layer which provides protectionagainst wear and a layer of oxide whose formation is hard to preventbecause of the very low equilibrium pressure of the oxide in anoxidising atmosphere. The depth of oxygen diffusion required to ensurewear resistance is some 20 micrometres. The minimum target depth forthis operation is therefore 35 to 50 μm.

Implementation of a process for furnace oxidising 3.5 to 4.6 m barswould require a furnace of sufficient size capable of working in anoxidising atmosphere at 800-1000° C.

The invention therefore also relates to the use of oxidising treatmentin a travelling arrangement at a higher temperature which ensures oxygendiffusion to a sufficient depth to provide wear resistance, maintaininga constant temperature—a measure of the uniformity of the oxidisedbar—without introducing straightness or mechanical inhomogeneities.Diffusion of oxygen over ˜50 μm may be obtained by induction heating at1300-1700° C. in an oxidising atmosphere consisting of argon and oxygen,at a rate of travel of 50-250 mm/min. Heating to a higher temperature islikely to cause phase changes in the metal (1725-1775° C.) or in theoxide (˜1700° C.). Travelling oxidation is carried out on absorber rodswhich have been welded to their bottom end plugs.

Furthermore, the bottom end plugs of welded tubes are treated to ensurecontinuity of protection against wear within the area of the bottom endplug of ogival shape. However, the processing of completed rods (withwelded top end plugs) is undesirable. In fact in some rods the presenceof packing pieces, a column of B₄C pellets and a supporting arrangementdisturbs heating, restricts the choice of packing pieces and supportingdevices (materials which are likely to give rise to molten eutectics atthe processing temperature must be ruled out). In addition to this, thechange in heating conditions at the hafnium-titanium junction isdifficult to control without risking excessive heating of the titanium,heating which would prejudice persistence of its mechanical properties.

Treatment in a travelling arrangement also makes it possible to avoidoxidising the part which will be welded to the top end plug, thusavoiding contaminating the weld.

The top end plugs of rods are protected against wear by treatment in astatic furnace in an oxidising atmosphere under conditions which ensurethat the alloy's properties are acquired. Treatment in a static furnaceis generally carried out at a temperature of between 550° C. and 850°C., for a period of 2 to 12 hours. For example treatment may be carriedout for 4 hours at 730° C.

Finally the invention also relates to a control cluster whose spider ismade of titanium-based alloy. This arrangement makes it possible tobenefit from the better mechanical properties of these alloys and theirlesser density. Thus design of the cluster becomes easier because partof the mass of the spider can be allocated to the absorber rods.

The spider supporting the absorber rods in the cluster may beconstructed to be of a shape and dimensions identical to those of thesupporting spiders for the absorber rods of control clusters accordingto the prior art. However, in some circumstances, depending upon theshape and size of the titanium alloy top end plug, the shape anddimensions of the parts of the spider to which the absorber rods areattached can be altered.

Instead of a steel supporting spider for the absorber rods, use of aspider made of titanium-based alloy makes it possible to benefit frommechanical properties of a higher grade than those of steel. Thereliability and service life of the spider can therefore be increased asa result of these improved mechanical properties. It is also possible toeffect a slight reduction in the transverse dimensions of the spidersupporting the control clusters when the spider is made oftitanium-based alloy having superior mechanical properties. The loss ofhead when the control cluster is lowered into the nuclear reactor coreand thus reduced, and the lowering time is also reduced.

The spider may be constructed by cutting it out from a piece of titaniumalloy whose metallurgical soundness has been checked. The risk ofdefects is thus reduced and the number of welded or brazed jointsbetween the constituent components of the spider is reduced. Cutting outmay be effected by mechanical, chemical or electrical machining, or bywater jet cutting.

Titanium alloys as envisaged above are not affected by corrosion withinthe nuclear reactor vessel. Fewer activatable products are thereforedelivered to the primary circuit.

Finally the metallurgical soundness of the material and the simplicityof manufacturing a spider from titanium-based alloy makes it possible toreduce manufacturing costs and operational abnormalities and increaseproductivity in the manufacture of control clusters.

A spider of titanium-based alloy can be used for any control clusterwhether or not it includes rods with hafnium tubes.

Tests have been performed on the mechanical strength of control clustersaccording to the invention under conditions reproducing the conditionsin a functioning nuclear reactor.

Wear tests have also been carried out on the different parts of absorberrods to validate anti-wear treatments using oxidation.

The tests performed were designed to check the wear resistance of theabsorber rod end plugs and in particular the top end plugs, the hafniumtubes of the absorber rods and the parts linking the top end plugs withthe control cluster spider. Endurance tests were carried out and showedthat control clusters according to the invention can operate within anuclear reactor without premature destruction for the envisaged servicelives of nuclear reactors according to the current art.

In order that the invention may be understood a control cluster and anabsorber rod according to the invention will be described by way ofexample with reference to the appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a control cluster for a pressurizedwater nuclear reactor inserted into a fuel assembly.

FIG. 2 is a view of an absorber rod according to the invention in axialcross-section.

FIG. 3 is a view of the top part of an absorber rod attached to the armof the spider, in partial cross-section.

DETAILED DESCRIPTION

In FIG. 1 a control cluster for a pressurised water nuclear reactor isindicated in general by the reference number 1.

Control cluster 1 comprises a bundle of absorber rods 2 and a spider 3supporting and holding rods 2 in the form of a bundle in which the rodsare parallel with each other and laterally positioned in the samearrangement as the fuel assembly guide tubes.

Spider 3 comprises a cylindrical hub 3 a with internal grooves so thatthe control cluster can be connected to an absorber rod in order to moveit in a vertical direction within the core and arms 3 b which are of onepiece with hub 3 a, to each of which absorber rods 2 are attached bytheir top end plugs.

Some at least of rods 2 in control cluster 1 comprise a tubular bodycomprising a hafnium tube.

In the case where cluster 1 is a black cluster, the tubes of allabsorber rods 2 in the cluster may be of hafnium.

In the case of a grey cluster, only some of rods 2 comprise a hafniumtube, the tubes of the other rods being of steel or any othernon-absorbent material which satisfies operational requirements within anuclear reactor.

FIG. 2 shows an absorber rod according to the invention in a blackcluster which can be used for example in a pressurized-water-coolednuclear reactor operating at a power of 1300 MWe.

Rod 2 illustrated in FIG. 2 comprises a hafnium tube 4 enclosing a stackof highly absorbent boron carbide B₄C pellets 5 which is sealed off atits upper extremity by a titanium alloy plug 6 and at its lowerextremity by a hafnium or zirconium alloy plug 7 of ogival shape. Oxygendiffusion 11 has been carried out on the tube welded to the bottom endplug and provides protection against wear. The top end plug may or maynot be protected by oxygen diffusion 12.

The hafnium used may contain more than 300 ppm of oxygen.

The stack of boron carbide B₄C pellets 5 is held within hafnium tube 4by a spring or any other immobilising device 8, the bottom end of thecolumn of pellets bearing against bottom end plug 7 through a strut 7 a.Bottom end plug 7 of hafnium rod 2 is made of one piece with the bottomend of hafnium tube 4 through a weld bead 7 b, welding being carried outfor example using a laser beam, a beam of electrons, TIG, friction orresistance welding. The weld obtained is sound and strong.

In accordance with the invention, top end plug 6 of rod 2 is made oftitanium or titanium alloy, for example Ti-6Al-4V (TA6V) alloy orTA3V2.5 alloy, and it is rigidly and leaktightly secured to the top endof tube 4 through a weld 9. Tests have demonstrated that the weldbetween titanium alloy plug 6 and hafnium tube 4 can be made using forexample a laser beam, an electron beam, TIG, friction or resistancewelding. The weld obtained is perfectly sound and perfectly strong. Inthe case of TIG or friction welds, the failure zone of ahafnium/titanium or hafnium/Zircalloy test piece lies outside the weldedzone. Failure occurs under a load corresponding to the ultimate strengthof the solid material.

As illustrated in FIG. 3, top end plug 6 of titanium alloy which securesabsorber rod 2 to an arm 3 b of spider 3 of the control cluster may beof a shape and dimensions which are identical to those of a top end plugof an absorber rod according to the prior art. The top end plug 6 whichhas a thread for securing the absorber rod to arm 3 b of spider 3 may beeither screwed into the spider arm or placed in a transverse positionand held by means of a top nut 10 which also guides the cluster when itis being raised.

As illustrated in FIG. 3, the top end plug of absorber rod 6 has a partof small cross-section 3 c which provides the rod with the requiredflexibility.

Furthermore, it has been established that weld 9 between titanium alloyplug 6 and the top end of hafnium tube 4 (FIG. 2) withstands mechanical,thermal and chemical stresses within a nuclear reactor environmentwithout any additional corrosion being observed at the connecting weld 9of top end plug 6.

Furthermore, when the control cluster is used in the core of a nuclearreactor, plug 6 lies above the top surface of the core, in a zone whichis not subjected to the intense neutron flux obtaining within thenuclear reactor core. The titanium alloy top end plug is therefore notsubjected to conditions giving rise to swelling under irradiation orloss of mechanical properties. The top end plug having high grademechanical properties thus retains its characteristics over long periodsof service within the core of a nuclear reactor.

Furthermore, the top end plug of hafnium absorber rods in the controlcluster according to the invention which is made of titanium alloyhaving high grade mechanical properties may be constructed in such a wayas to have the greatest possible length compatible with use of thecontrol cluster. The length of the hafnium tube can thus be reduced,making it possible to reduce the cost and adjust the mass of theabsorber rods.

The invention applies to any control cluster for a light water coolednuclear reactor comprising absorber rods comprising a hafnium tube.

1. A cluster for adjusting a pressurised water nuclear reactorcomprising a bundle of neutron-absorbing rods (2), each comprising ametal tube (4) called cladding which is sealed off at its upperextremity by a top end plug (6) and at its lower extremity by a bottomend plug (7) and a support (3), or spider, of radiating shape, to whichthe absorber rods (2) are attached through their top end plugs (6),characterised in that the cladding (4) of some at least of the absorberrods are weld-free hafnium tubes, the top end plugs (6) of the absorberrods (2) having hafnium cladding being of a titanium-based alloy andwelded to the part of the top extremity of the hafnium cladding (4) ofthe absorber rod (2), and the bottom end plugs (7) being of massivehafnium and being welded to the bottom extremity of the hafnium cladding(4) of the absorber rod (2).
 2. A cluster for adjustment according toclaim 1, characterised in that the top end plugs (6) of the absorberrods (2) having a hafnium tube (4) are of TA6V or TA3V2.5 titaniumalloy.
 3. A cluster for adjustment according to claim 1, characterisedin that protection against wear of the rods is provided by oxidation ata temperature of 1300° C. to 1700° C. in an oxidising atmosphere, withtravel at a rate of 50-250 mm/min over the cladding (4) welded to thebottom end plug (7).
 4. A cluster for adjustment according to claim 1,characterised in that protection against wear of the top end plugs (6)made of titanium alloy is obtained by treatment in a static furnace inan oxidising atmosphere under conditions ensuring that the properties ofthe titanium alloy persist.
 5. A cluster for adjustment according toclaim 4, characterised in that treatment in a static furnace is carriedout at a temperature of between 550° C. and 850° C. for a period ofbetween 2 and 12 hours.
 6. A cluster for adjustment according to claim1, characterised in that one at least of the top end plugs (6) andbottom end plugs (7) are welded using at least one of the followingprocedures: friction welding, resistance welding, TIG welding.
 7. Acluster for adjustment according to claim 1, characterised in that thehafnium used to manufacture the cladding (4) and the bottom end plugs(7) contains more than 300 ppm of oxygen.
 8. An absorber rod of acluster for the adjustment of a pressurised water nuclear reactor,characterised in that it comprises a cladding (4) of hafnium, a top endplug (6) of titanium alloy welded to an upper extremity of the hafniumcladding (4) and a bottom end plug (7) of massive hafnium welded to alower extremity of the hafnium cladding (4).
 9. A cluster for theadjustment of a pressurised water nuclear reactor, comprising a bundleof rods (2) and a support of radiating shape called a spider (3) towhich the absorber rods (2) are fixed through their top end plugs (6),characterised in that the spider (3) is made of titanium-based alloy.10. A cluster for adjustment according to claim 9, characterised in thatsome at least of the absorber rods (2) in the cluster (1) comprise ahafnium tube (4) and a top end plug (6) of titanium alloy welded to thetop extremity of the hafnium tube (4).
 11. Process for protecting anabsorber rod according to claim 8 against wear, characterised in thatthe cladding (4) of the absorber rod (2) is oxidised at a hightemperature in an oxidising atmosphere.
 12. Process for protectionaccording to claim 11, characterised in that oxidation of the cladding(4) welded to the bottom end plug (7) is carried out in a travellingarrangement at a temperature of 1300° C. to 1700° C. at a rate of 50 to250 mm/min.
 13. A cluster for adjusting a pressurised water nuclearreactor comprising: a bundle of neutron-absorbing rods, each comprisinga metal tube called cladding which is sealed off at an upper extremityby a top end plug and at a lower extremity by a bottom end plug and asupport of radiating shape, to which the absorbing rods are attachedthrough the top end plugs, wherein the cladding of at least one of theabsorber rods are weld-free hafnium tubes, the top end plugs of theabsorber rods having hafnium cladding of a titanium-based alloy andwelded to a part of the upper extremity of the hafnium cladding of theabsorber rod, and the bottom end plugs being of massive hafnium andwelded to the lower extremity of the hafnium cladding of the absorberrod.
 14. The cluster for adjustment according to claim 13, wherein thetop end plugs of the absorber rods having a hafnium tube are one or TA6Vand TA3V2.5 titanium alloy.
 15. The cluster for adjustment according toclaim 13, further comprising: oxidation on the rods, the oxidationprotecting against wear of the rods, the oxidation produced at atemperature of 1300° C. to 1700° C. in an oxidising atmosphere, withtravel at a rate of 50-250 mm/min over the cladding welded to the bottomend plug.
 16. The cluster for adjustment according to claim 13 furthercomprising: a titanium alloy on the top end plugs providing protectionagainst wear of the top end plugs, the titanium alloy obtained bytreatment in a static furnace in an oxidising atmosphere underconditions ensuring that properties of the titanium alloy persist. 17.The cluster for adjustment according to claim 16, wherein the treatmentin a static furnace is performed at a temperature of between 550° C. and850° C. for a period of between 2 and 12 hours.
 18. The cluster foradjustment according to claim 13, wherein at least one of the top endplugs and the bottom end plugs are welded using at least one of frictionwelding, resistance welding and TIG welding.
 19. The cluster foradjustment according to claim 13, wherein the hafnium used tomanufacture the cladding and the bottom end plugs contains more than 300ppm of oxygen.
 20. An absorber rod of a cluster for adjustment of apressurised water nuclear reactor, comprising: a cladding of hafnium; atop end plug of titanium alloy welded to an upper extremity of thehafnium cladding; and a bottom end plug of massive hafnium welded to alower extremity of the hafnium cladding.
 21. A cluster for adjustment ofa pressurised water nuclear reactor, comprising, a bundle of rods; and asupport of radiating shape called a spider to which the rods are fixedthrough top end plugs, wherein the spider is made of titanium-basedalloy.
 22. The cluster for adjustment according to claim 21, wherein atleast one of the absorber rods in the cluster comprise a hafnium tubeand a top end plug of titanium alloy welded to a top extremity of thehafnium tube.
 23. A process for protecting an absorber rod against wear,comprising: oxidizing a cladding of an absorber rod at a hightemperature above 1700° C. in an oxidizing atmosphere.
 24. The processfor protection according to claim 23, further comprising: oxidizing thecladding welded to the bottom end plug in a traveling arrangement at atemperature of 1300° C. to 1700° C. at a rate of 50 to 250 mm/min.
 25. Acluster for adjusting a pressurised water nuclear reactor comprising: abundle of neutron-absorbing rods, each comprising a metal tube calledcladding which is sealed off at an upper extremity by a top end plug andat a lower extremity by a bottom end plug and a spider of radiatingshape, to which the absorber rods are attached through the top endplugs, wherein the cladding of some at least of the absorber rods areweld-free hafnium tubes, the top end plugs of the absorber rods havinghafnium cladding of a titanium-based alloy and welded to a part of theupper extremity of the hafnium cladding of the absorber rod, and thebottom end plugs being of massive hafnium and welded to the lowerextremity of the hafnium cladding of the absorber rod.